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<p>指定类型或对象的<a href="object.html#.E5.AF.B9.E9.BD.90" title="cpp/language/object">对齐要求</a>。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<td class="t-sdsc-nopad"> <code><b>alignas(</b></code> <span class="t-spar">表达式</span> <code><b>)</b></code>
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<td class="t-sdsc-nopad"> <code><b>alignas(</b></code> <span class="t-spar">类型标识</span> <code><b>)</b></code>
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<td class="t-sdsc-nopad"> <code><b>alignas(</b></code> <span class="t-spar">包</span> <code><b>...</b></code> <code><b>)</b></code>
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<div class="t-li1"><span class="t-li">1)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas<span class="br0">(</span>表达式<span class="br0">)</span></span></span> 必须是求值为零或合法的<a href="object.html#.E5.AF.B9.E9.BD.90" title="cpp/language/object">对齐</a>或扩展对齐的<a href="constant_expression.html#.E6.95.B4.E5.9E.8B.E5.B8.B8.E9.87.8F.E8.A1.A8.E8.BE.BE.E5.BC.8F" title="cpp/language/constant expression">整型常量表达式</a>。</div>
<div class="t-li1"><span class="t-li">2)</span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas<span class="br0">(</span>alignof<span class="br0">(</span>类型<span class="br0">)</span><span class="br0">)</span></span></span></div>
<div class="t-li1"><span class="t-li">3)</span> 等价于对同一说明应用多个 alignas 说明符，逐个对应于<a href="parameter_pack.html" title="cpp/language/parameter pack">形参包</a>的各个成员，形参包可以是类型或非类型形参包。</div>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">alignas</span></span> 说明符可应用于变量或非位域类数据成员的声明，或可应用于 <a href="class.html" title="cpp/language/class">class/struct/union</a> 或<a href="enum.html" title="cpp/language/enum">枚举</a>的定义。它不能应用于函数形参或 catch 子句的异常形参。
</p><p>这种声明所声明的对象或类型的<a href="object.html#.E5.AF.B9.E9.BD.90" title="cpp/language/object">对齐要求</a>，将等于用于该声明的所有 <code>alignas</code> 说明符中最严格（最大）的非零 <span class="t-spar">表达式</span>，除非这会削弱类型的自然对齐。
</p><p>若某个声明上的最严格（最大）<code>alignas</code>，比当它没有任何 <code>alignas</code> 说明符的情况下本应有的对齐更弱（即弱于其原生对齐，或弱于同一对象或类型的另一声明上的 <code>alignas</code>），则程序非良构：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> alignas<span class="br0">(</span><span class="nu0">8</span><span class="br0">)</span> S <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> alignas<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> U <span class="br0">{</span> S s<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 错误：若无 alignas(1) 则 U 的对齐将为 8</span></pre></div></div>
<p>无效的非零对齐，例如 <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span></span></span> 非良构。
</p><p>同一声明上，弱于其他 <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas</span></span> 的有效的非零对齐被忽略。
</p><p>始终忽略 <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span></span></span>。
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p>按 ISO C11 标准，C 语言有 <span class="t-c"><span class="mw-geshi cpp source-cpp">_Alignas</span></span> 关键词，并于头文件 <a href="../../c/types.html" title="c/types"><tt>&lt;stdalign.h&gt;</tt></a> 中将 <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas</span></span> 定义为展开成该关键词的预处理器宏，但 C++ 中这是关键词，且头文件 <a href="../header.html#.E6.97.A0.E6.84.8F.E4.B9.89.E7.9A.84_C_.E5.A4.B4.E6.96.87.E4.BB.B6" title="cpp/header"><tt>&lt;stdalign.h&gt;</tt></a> 与 <a href="../header/cstdalign.html" title="cpp/header/cstdalign"><tt>&lt;cstdalign&gt;</tt></a> 并不定义这个宏。不过它们仍定义宏常量 <span class="t-c"><span class="mw-geshi cpp source-cpp">__alignas_is_defined</span></span>。
</p>
<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
<p><a href="../keyword/alignas.html" title="cpp/keyword/alignas"><tt>alignas</tt></a>
</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// sse_t 类型的每个对象将对齐到 16 字节边界</span>
<span class="kw1">struct</span> alignas<span class="br0">(</span><span class="nu0">16</span><span class="br0">)</span> sse_t
<span class="br0">{</span>
  <span class="kw4">float</span> sse_data<span class="br0">[</span><span class="nu0">4</span><span class="br0">]</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// 数组 "cacheline" 将对齐到 128字节边界</span>
alignas<span class="br0">(</span><span class="nu0">128</span><span class="br0">)</span> <span class="kw4">char</span> cacheline<span class="br0">[</span><span class="nu0">128</span><span class="br0">]</span><span class="sy4">;</span>
 
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<p><br>
</p>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <a href="alignof.html" title="cpp/language/alignof"> <code>alignof</code> 运算符</a>
</td>
<td>  查询类型的对齐要求 <span class="t-mark-rev t-since-cxx11">(C++11 起)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../types/alignment_of.html" title="cpp/types/alignment of"> <span class="t-lines"><span>alignment_of</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td>   获取类型的对齐要求  <br> <span class="t-mark">(类模板)</span> </td></tr>

</table>

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